Puny one (4)
I believe the answer is:
runt
'puny one' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for runt that I've seen before include "Litter-weakling" , "Smallest squealer" , "Smallest piglet of a litter" , "Weakling of the litter" , "Littlest oinker" .)